When I was younger, I would only take Vitamin C tablets when I was already sick. As I grew older, though, I began to take the tablets daily, in an effort to boost my immune system and stave off impending sickness.
I take one 1000mg Vitamin C tablet, twice daily. The GNC tablets include bioflavonoids, which I have no idea of their purpose, and rose hips. They are also a timed release formula, so more of the vitamins should be absorbed by your body, instead of being expelled.
The tablet size can be a bit large, and while I have no problem with larger pills, if you are squeemish, I would pass. Additionally, have water immediately ready; I often take pills without water, and these immediately start to leave an acidic, metallic taste in your mouth, which is hard to wash out.